The invention relates to an operating device for one or more low-pressure discharge lamps having filaments. In particular, it relates to a circuit which detects the breakage of a filament of a lamp and disconnects the operating device.
The service life of a low-pressure discharge lamp fitted with filaments is determined chiefly by the service life of the filaments. If the filaments are consumed, there is firstly an increase in the lamp voltage, accompanied by an undesired temperature increase in the filament region of the lamp. The lamp also mostly shows a rectifying effect at this stage. Finally, the filament breaks, and this can lead to destruction of the lamp operating device and to a dangerous overheating of the ends of the lamp. Some disconnection devices are known for reliable operation of the lamp and to protect the operating device.
It has also emerged that monitoring the filaments with regard to breakage suffices in order to be able to ensure reliable operation of the system of lamp and operating device. In known solutions, it is detected whether a DC test current can flow through the filaments to be tested (DE 3805510). The disadvantage of this method is that the test current flows in addition to the current required for normal operation, and thus constitutes an additional load for the filaments.
Also obvious is the use of an AC test current. For this purpose, the current supply for the gas discharge is conducted via in each case only one terminal of the filaments. The respective other terminals of the filaments are bridged by a capacitor (termed resonance capacitor below). This resonance capacitor is mostly also used to generate the starting voltage, and therefore does not constitute an additional outlay on components. The current for the gas discharge is provided by an AC voltage generator. This current is now divided into a portion which flows through the gas discharge path and a portion which flows through the resonance capacitor. In the case of filament breakage, the current component through the resonance capacitor vanishes. In order to disconnect the operating device in the case of filament breakage, it is therefore necessary to monitor the current through the resonance capacitor. It is advantageous to be able to evaluate this current in a potential-free fashion. U.S. Pat. No. 5,952,832 proposes a transformer whose primary winding is connected in series with the resonance capacitor. It is now possible on the secondary side of the transformer to evaluate the current through the resonance capacitor in a potential-free fashion. However, the use of a transformer signifies a substantial outlay on cost.
It is the object of the present invention to provide as cost-effectively as possible a potential-free evaluation of the current through the resonance capacitor for the purpose of disconnecting the operating device in the event of filament breakage.
As a rule, the operating device includes an AC voltage generator which feeds energy into the load circuit. The principle of such an arrangement is illustrated in FIG. 1. The series circuit of the lamp reactor L1 and the lamp Lp is connected to the two terminals of the AC voltage generator G. A filament terminal is used in each case to connect the lamp Lp. The resonance capacitor C1 is connected to the respective other filament terminal. Describing the lamp by an equivalent load resistor R1 yields the following expression for the load circuit impedance Z as a function of the complex frequency s:       Z    ⁡          (      s      )        =                    R        1            +              sL        1            +                        s          2                ⁢                  L          1                ⁢                  C          1                ⁢                  R          1                            1      +                        sC          1                ⁢                  R          1                    
The phase characteristic of this expression is plotted in FIG. 2 against the technical frequency. The resonance capacitor C1 is the parameter. The value of its capacitance is 10 nF or 10 pF. R1 has a resistance of respectively 500 ohms, and L1 respectively has an inductance of 2 mH. 500 ohms is the typical value for the equivalent resistance of a compact fluorescent lamp, while 2 mH represents a typical value for the inductance of a lamp reactor suitable for operating this lamp. For this arrangement, a value of 10 nF is suitable for the capacitance of the resonance capacitor. In accordance with FIG. 2, a phase angle of approximately 70xc2x0 is yielded for the load circuit impedance given an operating frequency of 50 kHz. If a filament now breaks, the resonance capacitor is disconnected from the load circuit. A value of 10 pF can be assumed as residual capacitance, which is essentially formed by the wiring. In accordance with FIG. 2, it follows that in the case of a broken filament a phase angle of approximately 50xc2x0 results for the load circuit impedance. A phase detector which triggers a disconnection of the operating device now suffices for detection as claimed in the invention if the phase of the load circuit impedance drops by a prescribed value.
A further cost-effective possibility for potential-free detection of a filament breakage is yielded by the use of an optocoupler. The current through the resonance capacitor or a part thereof is conducted through the light emitting diode (input) of the optocoupler. This light emitting diode is extinguished in the case of filament breakage. This can be detected in the potential-free fashion at the output of the optocoupler and trigger disconnection of the operating device.